Optical Studies of Shallow Acceptors in CdS and CdSe

Abstract

We show that Li and Na are the only soluble substitutional acceptors in CdS and CdSe with relatively small binding energies. A shallow acceptor is observed for P doping, but it arises from a complex. These results, and also the fact that the substitutional P is a deep acceptor in the sulfides and selenides, but not in the tellurides, are consistent with the trends in acceptor binding energy observed in other semiconductors. We present evidence that a native double donor exists in CdS, but that Li and Na acceptors are compensated by ordinary donors and not by isolated double donors. Analysis of thermodynamic data shows that isolated native donors should not be effective in compensating acceptors in ZnSe, CdSe, or CdTe. The compensating donors in CdS are found to be unusually shallow, indicating that they may be Li or Na interstitials. An attempt to drift out the Li-interstitial donors with an applied electric field is discussed.